1. Field of the Invention
The present invention relates to a method for grinding semiconductor wafers.
2. Background Art
Semiconductor wafers are produced according to the prior art by a plurality of process groups:
a) producing a monocrystalline semiconductor ingots (crystal growth)
b) slicing the ingot into individual wafers (“wafering”, “sawing”)
c) mechanical processing
d) chemical processing
e) chemical-mechanical processing
f) optionally coating.
In addition to the foregoing groups, a multiplicity of further steps, such as cleaning, sorting, measuring and packaging steps, may also be carried out.
The group of mechanical processing steps comprises rounding the wafer edge and planarizing the wafer surface by means of mechanically abrasive steps which remove material. Edge rounding is carried out by grinding or polishing, for example with round or band-shaped tools.
Planarizing of the wafer surface is carried out “in batch”, i.e. for a plurality of wafers simultaneously, by so-called “lapping” with free abrasive using a lapping suspension (“slurry”) or as a single-wafer process by grinding with bound abrasive.
In the case of single-sided grinding, one side of the semiconductor wafer is fixed by means of a vacuum in a wafer carrier (“chuck”), and the other side is processed by a grinding disc coated with grinding abrasive. If both sides of the wafer are intended to be ground, the processing of the two sides of the semiconductor wafer is generally carried out sequentially.
Batch double-sided grinding methods are also employed, with lapping kinematics in which there is bound abrasive or abrasive applied onto coatings (cloth) onto large working discs facing one another, between which the semiconductor wafers are ground on both sides as in the case of lapping while being partially free to move in guide cages.
In order to achieve a particularly good geometry of the processed wafers, a simultaneous double-sided grinding method (“double disc grinding”, DDG) is often used.
EP 1 049 145 A1 discloses a processing sequence which involves a DDG pregrinding step (“roughing”), followed by one or more (sequential) single-sided fine grinding steps (“flattening”).
Conversely, U.S. Pat. No. 6,066,565 describes the use of the DDG method in a two-stage process with double-sided pregrinding and a double-sided fine grinding. This requires two machines and clamping the workpiece a plurality of times.
DE 101 42 400 A1 discloses a method which is carried out with a simultaneous double-sided grinding machine and is characterized in that it comprises only a single processing operation with the workpiece being clamped only once. This means that the generally required pre-processing and fine processing (“roughing” and “flattening”) take place in a single integrated processing step. Also described is a simultaneous double-sided grinding method, using a workpiece retainer which holds and moves the semiconductor wafer virtually without constrained guiding (“free-floating process”, FFP).
In the case of simultaneous double-sided grinding, which is also described for example in EP 868 974 A2, the semiconductor wafer is processed simultaneously on both sides while floating freely between two grinding discs mounted on opposite collinear spindles, and is guided substantially free from constraining forces axially between a water cushion (hydrostatic principle) or air cushion (aerostatic principle) acting on the front and back sides, and is radially prevented from floating thereon loosely by a thin circumferential guide ring or by individual radial spokes. The semiconductor wafer rotates about its symmetry axis during the grinding. This rotation is driven by friction tools engaging on the front and back sides, via a “notch finger” which engages in an orienting reference “notch”, or by friction belts which partially enclosed the semiconductor wafer circumferentially.
DE 10 2004 005 702 A1 discloses a method for producing a semiconductor wafer, comprising double-sided grinding of the semiconductor wafer in which the semiconductor wafer is ground initially coarsely and subsequently finely on both sides by a grinding tool, during which the semiconductor wafer remains clamped in the grinding machine between coarse and fine grinding, and the grinding tool engages with an essentially constant load when changing from course grinding to fine grinding. DE 10 2004 005 702 A1 furthermore describes a device for the double-sided grinding of flat workpieces, comprising two double spindles each with an inner sub-spindle and an outer sub-spindle, an instrument for loading and unloading the workpiece, and a workpiece retainer which is arranged between the double spindles and by which the workpiece is held free-floating during grinding. The sub-spindles are arranged coaxially and carry grinding tools for grinding opposite sides of the workpiece, at least one sub-spindle of each double spindle being axially displaceable independently of the other sub-spindle of the double spindle.
During the grinding processes, whether single-sided or double-sided grinding, it is necessary to cool the grinding tool and/or the processed semiconductor wafer. Water or deionized water is conventionally used as a coolant. Commercial grinding machines, for example the models DFG8540 and DFG8560 (“Grinder 800 Series”) from Disco Corp., which are suitable for grinding wafers with diameters of 100-200 mm and 200-300 mm respectively, are equipped on the working side with a vacuum unit which ensures a constant coolant flow rate of 1 or 3 l/min (=litres per minute) during the grinding, depending on the coolant temperature (constantly 1 l/min for a temperature of less than 22° C., constantly 3 l/min for a temperature of more than 22° C.).
Double-sided grinding machines are available for example from Koyo Machine Industries Co., Ltd. The model DXSG320 is suitable for the DDG grinding of 300 mm wafers. Both vertical and horizontal spindles are employed in combination with special diamond grinding tools. These grinding tools are designed so that they cut only with the edge and combine a rapid forward feed rate with little heat production. The main difference is the wafer holding. The wafer to be processed is fixed by hydrostatic pressure pads on both sides in a transport ring. The wafer is driven merely by a small nose which engages in the notch or in the flat. Stress-free holding of the wafer can be ensured in this way.
JP58143948 describes a method for cooling in single-sided grinding machines and the way in which the coolant is applied onto the wafer surface to be processed. JP2250771 teaches to determine the coolant flow rate and, depending on a measured grinding temperature, to increase it rapidly in order to both keep the grinding temperature within a predetermined temperature range and to keep the amount of coolant used at the minimum required level.
In double-sided grinding machines, the process coolant conventionally emerges from the centre of the grinding tool and is transported to the grinding teeth by means of the centrifugal force. The coolant throughput can be regulated by keeping the coolant flow rate at a setpoint value. This regulating may be carried out electronically by means of a suitable measuring device and actuators or by mechanical means (pressure reducer).
US 2001/025660 AA proposes to monitor machine working/engagement times and machine idle/setup times (“active” and “idle” modes) of the grinding machine automatically and to regulate the coolant flow rate accordingly. At the start of the setup time the coolant flow is reduced or entirely stopped, and it is subsequently increased periodically during the setup time, i.e. the introduction of a new workpiece. This achieves more economical use of coolant compared with the solutions likewise known in the prior art, namely to keep the coolant flow rate constant even during the machine idle time. A certain coolant flow rate at least towards the end of the setup time seems to be advantageous, since the grinding machines contain sensors which are extremely sensitive to temperature differences.
U.S. Pat. No. 5,113,622 AA proposes temperature detectors on the coolant in- and outflows. The difference between the temperatures at the inflow and the outflow is thereby determined. This temperature difference is intended to indicate the amount of heat generated during the grinding, by taking into account the coolant flow rate and the dissipation of heat. In order to keep the temperature of the GaAs wafer to be processed below a particular target temperature, and to avoid fracture or warp due to residual thermal stresses, corresponding regulation of the coolant flow rate is proposed as a function of the continuously determined amounts of heat.
Known prior art methods therefore involve either keeping the temperature of the workpiece constant or below a target value and increasing the coolant flow rate accordingly, or setting a constant coolant flow rate with one or two target values. Problems, which the prior art was not able to resolve at the application date, consist of workpieces ground with one and the same grinding tool having different surface damages, which implies non-constant grinding conditions, and also the lifetime of grinding tools. In the latter context, one skilled in the art recognizes that grinding tool service life is generally unsatisfactory even when a constant coolant flow rate and (therefore) supposedly sufficient cooling of the workpiece and the grinding tool are meant to be ensured.